Short-ish answer:
Scoutbook and IA2 both look at the same underlying advancement database, but using different user interfaces.
The Scoutbook interface is reached from https://scoutbook.scouting.org. It’s a different interface, so it has slightly different options. When the unit leader logs in, they would go to:
My Dashboard → Administration → My Units → Troop ### → Troop Roster → Scout’s Name → Scout’s Advancement
Scroll down to the merit badges section. If CiS is already listed, click on the name. If not, go to Start a New Merit Badge near the bottom, and start CiS.
Once in the CiS MB, the leader can click on the checkbox for Completed and set a date, and on Leader Approved.
Long-ish Answer:
There are a lot of help and training pages here:
In particular, I’d recommend these for unit scouters getting started with Scoutbook:
- Getting a Unit Started in Scoutbook - Scoutbook Knowledge Base
- Scoutbook for Leaders (Videos) - Scoutbook Knowledge Base
- Feature Assistant - What is it?
- Troop Advancement Using Scoutbook (Video) - Scoutbook Knowledge Base
- Cub Scout Advancement Using Scoutbook (Video) - Scoutbook Knowledge Base
- Granting Roles and Permissions in Scoutbook (SB) - Scoutbook Knowledge Base
- Using Scoutbook Calendars (SB) - Scoutbook Knowledge Base
- Using Scoutbook to Communicate With Your Unit - Scoutbook Knowledge Base
and this one for parents getting started with Scoutbook: